In our laboratory, we have cloned and expressed the HuIFNA21 gene in a pQE-30 expression system. Binding studies using I-125 IFN alpha 2b have shown that the A21 protein expressed from the above gene competes poorly for IFN-alpha 2b sites on both Dauda and AU937 cells. The data suggest that there may be two different receptor bindimg sites on the surface of the cells: one that can only bind the IFN-alpha 2b and one that can bind both IFN-alpha 2b and IFNA21. We are, therefore , interested in what domain of the IFNA21 is responsible for this distinctive binding site and we will make this determination by transferring a heterologous segment of the IFNA21 into Interferon-alpha 2. This clone has been constructed using a pQE-30 system. It is now necessary to express the recombinant DNA and ascertain whether the resulting protein has the same type of binding profile as IFNA21.